Surface mount technology (SMT) is a mounting process where electronic components, such as integrated circuits (ICs), are mounted or placed directly onto the surface of printed circuit boards (PCBs). The SMT process may result in electronic component defects that can escape existing inspection and test processes, and may result in early life failure of systems using these defective electronic components. One type of damage is related to moisture sensitive devices, which include most ICs made with plastic or organic materials. This problem has been observed and documented since the early days of SMT technology and there are industrial standards that dictate the proper procedures. However, this failure often goes undetected.
Moisture sensitive devices are electronic components that absorb moisture and have a high potential for internal cracking during the assembly process such as during high temperature solder reflow process. All IC components are classified as moisture sensitive devices. Such electronic components are encapsulated with plastic compounds and other organic materials. Moisture from atmospheric humidity enters permeable packaging materials through diffusion. The moisture typically collects at dissimilar material interfaces within the packaging materials.
The electronic components are packaged prior to mounting on the PCB. During the SMT process, contacts on the electronic component are mounted to corresponding contact pads on the surface of the PCB. An electronic component may include short pins or leads of various styles, flat contacts, a matrix of solder balls (BGAs), or terminations on the body of the electronic component for interconnecting with the corresponding contact pads on the PCB. The contacts are connected to the contact pads using a solder reflow process. The high temperatures involved in vapor phase or reflow soldering may cause the absorbed moisture to expand rapidly, possibly causing internal stress known as “Popcorning” that causes package cracking. Surface peeling between the die pad and the resin is may also be caused by increased water vapor pressure. Surface delamination is may also result due to materials mismatch shear strain on bond wires and wire necking that leads to micro-cracking extending to the outside of the package. These internal defects due to moisture are nearly impossible to detect during the PCB assembly and test process. It is understood that such internal defects may be the result of any higher temperature processing step that is performed on a packaged electronic component.
According to the industrial standard, J-STD-033, Handling, Packing, Shipping and Use of Moisture/Reflow Sensitive Surface Mount Devices, a moisture sensitive device needs to be stored in an environment below 5% humidity to avoid moisture absorption and related thermal shock damage during the reflow process. To minimize moisture absorption, an exposure time of the moisture sensitive device must be controlled, where the exposure time is the time during which the moisture sensitive device is exposed to a higher humidity environment. It is a challenge to control the exposure time. Tracking exposure time involves a lot of paper work and handling. There is a software available in the market to keep track of the exposure time. However, if the exposure time is exceeded, then a baking process may be required to remove the excess moisture from the device. In addition to the cost associated with the baking process equipment, performing of the actual baking process will increase the risk of component oxidation.
A low humidity storage environment minimizes the moisture that can be absorbed and in some cases can remove some of the already absorbed moisture from the moisture sensitive device. A conventional low humidity storage environment is a dry storage box made of moisture diffusion-resistant walls. Inside the dry storage box is a desiccant. The moisture resistant device is placed inside the dry storage box. A downside is that the moisture resistant device must stay within the dry storage box for a relatively long time period, in some applications 2-3 hours. Another downside is that the desiccant periodically becomes saturated and either needs to be replaced or have the moisture removed. This results in a down period where the dry storage box can not be used for storing packaged electronic devices.
Another conventional low humidity storage environment is a nitrogen/dry air purge cabinet. Nitrogen does not absorb moisture and can function as a barrier to moisture. As such, the nitrogen protects the packaged electronic component from absorbing moisture outside the cabinet. However, any moisture already absorbed by the packaged electronic component cannot escape as the nitrogen forms a barrier around the package. Further, since the already absorbed moisture can not escape, the moisture will move toward the electronic component within the package. In this case, any subsequent cracking due to vapor expansion will occur near or at the electronic component, which is undesirable.
Yet another conventional low humidity storage environment is a vacuum sealed moisture barrier bag. Packaged electronic components are placed within the bag, along with a desiccant, and the bag is vacuum sealed. For low volume manufacturing, the bag is opened and one or more packaged electronic components are removed for assembly, while the remaining packaged electronic components are resealed in the bag, typically with a new desiccant. Some bags may not be resealable and a new bag may be needed. This is a time consuming and expensive process.